Drive mechanism for radial adjustment of a drilling tool

ABSTRACT

The output shaft of a rotary drive motor is connected to a screw shaft which is disposed on the same axis as the output shaft. The screw shaft is rotatably mounted in an axial bearing in a housing, and engages an internal screw element (nut) fixed to a thrust body which is axially displaceable in the housing. A drive rod is rotatably mounted in the thrust body. In order to reduce the longitudinal dimension of the device, the free end of the screw shaft extends into a bore hole in the end of the drive rod, which rod serves to axially displace a thrust bar for adjusting a drilling tool in a drill head.

BACKGROUND AND SUMMARY OF THE PRESENT INVENTION

The invention relates to a drive mechanism for radial adjustment of adrilling tool in a drill head by means of an axially moveable drive rodwhich is connected to a rotary drive motor via a screw drive mechanism.

The radial adjustment of a drilling tool in a drill head has the purposeof separating the drilling tool from the surface being cut, at theconclusion of a drilling operation, in such a way that no scores orgrooves are left [i.e., produced] in said surface. The mechanism alsoserves to produce a radial adjustment of the drilling tool to compensatefor cutting wear occurring on the tool. This compensation enables theoperating life of a drilling tool to be greatly extended, because ingeneral the limiting condition in tool life is the point where, due tocutting wear, the dimensions of the surface being cut are no longerwithin the acceptable range.

Most types of mechanism for radial adjustment of a drilling tool in adrill head have the feature that a drive rod, which is centrallydisposed in the drill head and extends through the tool spindle holdingwhich spindle holds the drill head, acts directly or indirectly on thedrilling tool via an inclined surface, whereby the tool is radiallyadjusted. The drive rod rotates with the drill head and the toolspindle. The drive rod is driven axially by a rotary drive motor via ascrew drive which converts the rotational movement produced by the motorto an axial thrust movement.

In a known drive mechanism of the type initially described above (Ger.Auslegeschrift 21 67 082), the rotary drive motor is connected to thescrew shaft via a rotating coupling which permits longitudinal movement.The internal screw which engages the screw shaft is fixed to themounting. The end of the screw shaft which is distant from the rotarydrive motor is connected to the drive rod via a thrust bearing.

There are embodiments of the type of drive mechanism described initiallyabove in which the rotary drive motor is not aligned with thelongitudinal axis of the screw shaft but with an axis intersecting thelongitudinal axis (U.S. Pat No. 3,286,556) or with an axis parallel tothe longitudinal axis (Fr. Pat. No. 2,327,840). In contrast to these,the known device of Ger. AS 21 67 082 (cited supra) has relatively smalllateral dimensions (diameter); however, the axial length is relativelylarge. A significant factor in this is the fact that the achievableaxial thrust movement is reflected twofold in the axial length of thedrive mechanism--first in the rotating coupling, in which allowancesmust be made for this axial movement, and secondly in the length of thescrew shaft which extends beyond the internal screw, which length alsomust equal the achievable axial movement. Further, there is adegradation of the adjustment accuracy of the known drive mechanism dueto the fact that there is play in the required rotating coupling, atleast when unavoidable wear develops.

An object of the invention is thus to devise a drive mechanism of thetype described initially above, such that the mechanism has same-axisconstruction, and thereby small lateral dimensions, but is also as shortas possible in its axial dimension, and further is comprised of types ofelements which in combination result in very high tool adjustmentaccuracy.

This object and others are achieved according to the invention in that ascrew shaft which is connected to and disposed on the same axis as theoutput shaft of the rotary drive motor is rotatably mounted in an axialbearing in a housing, and engages an internal screw element. Theinternal screw element is fixed to a thrust body which is axiallydisplaceable in the housing. A rotatable drive rod which can beconnected to the thrust bar in the tool is rotatably mounted in an axialbearing in the thrust body. The free end of the screw shaft extends intoa bore hole in the end of the drive rod.

The types of working elements used, namely axial bearings and a screwdrive, make possible an embodiment of the drive mechanism with very highadjustment accuracy. The axial dimension is relatively short, due to thefact that the necessary axial adjustment distance is reflected only oncein the axial dimension of the drive mechanism, namely at the locationwhere it is necessary to provide room for the axial movement of thethrust body. Although the part of the screw shaft which projects fromthe internal screw must also be at least as long as the prescribeddisplacement distance for effecting the adjustment, in the inventivemechanism it does not add to the overall axial dimension of the drivemechanism because the free end of the screw shaft extends into the borehole in the end of the drive rod.

In a particularly advantageous embodiment of the invention the axialbearing of the drive rod is disposed axially adjacent to the internalscrew element and surrounds the end of the drive rod. In this way theaxial segment of the mechanism in which the free end of the screw shaftextends into the bore hole of the drive rod is also simultaneouslyemployed to accommodate the the axial bearing for the drive rod, so thatno additional axial space is taken up in mounting this axial bearing.The sole condition which determines the length of the thrust body is theneed to dispose the internal screw element and the axial bearing for thedrive rod axially adjacent to each other.

A particularly advantageous embodiment of the invention from themanufacturing engineering standpoint is characterized in that the thrustbody is axially displaceably guided in a bore in the housing. The axialbearing of the screw shaft is mounted in an insert which is held in thebore in the housing. Under this arrangement the housing comprisesessentially a cylinder with a longitudinal bore, which can bemanufactured with very high precision. The thrust body is guided in thelongitudinal bore, and also the axial bearing of the screw shaft is heldin an insert which is mounted in the bore. Thus in addition thearrangement greatly simplifies the assembly of the drive mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail hereinafter, with the aidof an exemplary embodiment which is illustrated in the drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawing is a longitudinal cross section of a drive mechanism forradial adjustment of a drilling tool in a drill head (not shown). Athrust body 3, having sliding bushings or sleeve bearings 4 on bothends, is axially displaceably housed in a housing 1 which has acylindrical longitudinal bore 2. An adjusting spring 5 which is screwedonto the thrust body 3 is guided in a longitudinal slot 6 in housing 1,which slot is closed externally by a cover 7.

A drive rod 9 is rotatably mounted in an axial bearing in a recessedbore hole 8 in the thrust body 3, which axial bearing comprises twoaxially braced ball or roller bearings 10.

The drive rod 9 extends through a hole 11 in an end plate 12 which isscrewed onto housing 1. On the outer end of rod 9 there is a threadedextension 13 by which it can be attached to an axially displaceablethrust bar on the drill head. Drive rod 9 has a flange 14 at its otherend, against which the inner race of one of the axial bearings 10 lies.The inner race of the other axial bearing 10 is held by a nut 15 screwedonto the drive rod 9. A ring nut 16 screwed into the bore hole 8 bracesthe outer races of bearings 10 against each other without any play.

On the end of the housing 1 which is distant from drive rod 9 a recessedinsert 17 is mounted in the longitudinal bore 2. Rotary drive motor 18,which may be a stepping motor, for example, is screwed onto this insert.The output shaft 19 of the motor 18 engages a blind hole 20 at the endof a screw shaft 21. An adjusting spring 22 effects torque transmissionfrom the output shaft 19 to the screw shaft 21.

An axial bearing comprising two ball or roller bearings 24 is mounted bya ring nut 25 in a bore hole 23 of insert 17. A nut 26 holds thebearings 24 on the screw shaft 21. A screw segment 27 of screw shaft 21extends through an internal screw element 28 which is fixed in thethrust body 3 between a flange 29 on the end of thrust body 3 which andfaces the motor and a ring insert 30, which ring insert is held in borehole 8 by the force of the outer race of the axial bearing 10.

The free end of the screw segment 27 of the screw shaft 21 extends intoa bore hole 31 in the end of drive rod 9 which faces the motor.

As may be seen from the drawing, the motor 18, screw shaft 21, and driverod 9 are disposed along the same axis.

The internal screw element 28 engaged by the screw segment 27 of screwshaft 21 may comprise, e.g., two nuts which are axially braced againsteach other in order to eliminate play. The screw shaft 21 and associatedinternal screw element 28 may alternatively be in the form of arecirculating ball bearing screw and nut or (as in the embodimentillustrated) a planetary worm and screw-roller mechanism. In the lattermechanism there is a plurality of screw rollers mounted planetarilyaround the screw segment 27 of screw shaft 21, which screw rollerssimultaneously engage the screw segment 27 of screw shaft 21 and aninternal screw of internal screw element 28.

When the motor 18 drives the screw 21 in a rotational motion, theinternal screw element 28 and with it also the thrust body 3 and thedrive rod 9 are displaced axially outward (to the left in the drawing),until the end face of the thrust body 3 reaches the position of thecover 12 which position is indicated by the dot-dashed line. In theprocess the thrust body 3 is guided axially in the housing 1 withoutrotation. Drive rod 9, which is rotatably mounted in the thrust body 3,is rotated with the tool spindle which bears the drill head.

The bearing in the motor 18 may be used as the axial bearing for thescrew shaft 21, as an alternative instead of the separate axial bearing24. This results in further shortening of the drive mechanism.

The principles, preferred embodiments and mode of operation of thepresent invention have been described in the foregoing specification.However, the invention which is intended to be protected is not to beconstrued as limited to the particular embodiments disclosed. Theembodiments are to be regarded as illustrative rather than restrictive.Variations and changes may be made by others without departing from thespirit of the present invention. Accordingly, it is expressly intendedthat all such variations and changes which fall within the spirit andscope of the present invention as defined in the claims be embracedthereby.

What is claimed is:
 1. In a drive mechanism for effecting radialadjustment of a drilling tool in a drill head, said drive mechanismcomprising a housing, a thrust body mounted in said housing for axialsliding movement therein, a first axial bearing carried by said thrustbody, a rotatable drive rod rotatably mounted in said first axialbearing for rotation about a longitudinal axis, said drive rod beingmovable along said longitudinal axis along with said thrust body betweenlongitudinally inner and outer positions for radially adjusting adrilling tool, a second axial bearing disposed in said housing, a screwshaft mounted in said second bearing for rotation about saidlongitudinal axis, means preventing longitudinal movement of said screwshaft, said screw shaft including an externally threaded extensionextending along said longitudinal axis toward said drive rod, aninternally threaded element carried by said thrust body and beingnon-rotatable relative thereto, said threaded element being threadedlymounted on said threaded extension, a drive motor operably connected tosaid screw shaft to rotate the latter about said longitudinal axis forproducing longitudinal movement of said threaded element along saidextension to longitudinally move said drive rod, the improvement whereinan end of said drive rod facing said screw shaft has an outer diameterlarger than that of said extension and includes a bore disposed in saidend, said bore extending along said longitudinal axis in alignment withsaid extension to telescopingly receive said extension when said driverod is in its longitudinally inner position.